The present invention relates to an infrared radiation detector such as, for example, might be used on a heat seeking missile. See, for example, British patent 1,355,326, published June 5, 1974 and U.S. Pat. No. 3,323,757, dated June 6, 1967. Its principal purpose is to reduce spurious radiation impinging on the detector cell.
German patent specification 1,223,585 discloses a detector device responsive to infrared radiation comprising a pot-shaped double-walled vessel the inner wall of which forms a cooling container and, together with the outer wall, defines a vacuum chamber. A detector element to be exposed to infrared radiation is located within the vacuum chamber and in heat-conducting connection with the cooling means container, said detector element being exposed to the infrared radiation in an imaging path of rays through an end of the outer wall. An opaque screen is located in the vacuum chamber, said screen enclosing the detector element and defining a radiation entrance aperture. The screen is also in heat conducting connection with the cooling means container. The purpose of the cooling means container and of the screen cooled thereby is to improve the signal-to-noise ratio of the detector by keeping this detector itself at a low temperature and to shield the detector from the radiation emitted by the elements of the apparatus containing the detector device. This prior art detector device suffers from the disadvantage that the detector is exposed to radiation not only from the imaging path of rays but also from lateral areas outside this imaging path of rays as illustrated in FIG. 1 of German patent specification 2,104,561 by hatched areas. It has been found that this radiation may also result in undesirable noise level.
German patent specification 2,109,561 discloses a detector device of this type in which the detector is to be shielded from all that radiation not emanating from the imaging path of rays. In accordance with the teaching of this German patent specification this is achieved in that optical imaging means are arranged in the radiation entrance aperture of the opaque screen, said imaging optical system being arranged to image either a field of view or (preferably) a pupil of the imaging path of rays on the detector element. In one embodiment of German patent specification 2,109,561 the imaging path of rays comprises a Cassegrain system which consists of an annular concave mirror facing towards the field of view to be observed and a plane mirror arranged to receive the radiation from the concave mirror and to image the field of view in an image plane. In the prior art device a partly transparent and partly opaque reticle is located in this image plane. The reticle may, for example, be of the type shown and described in British patent 1,355,327.
In the prior art device the reticle is located in the path of rays in front of the end face of the outer wall of the cylindrical, double-walled vessel, which outer wall encloses the vacuum chamber. The inner wall of the vessel, which wall is coaxial with respect to the outer wall and has its end face axially spaced from the end face of the outer wall, encloses the cooling means container. The detector element is attached to the end face of the inner wall. The screen forms a funnel having a reflecting internal surface, and the radiation entrance aperture of the screen or funnel is closed by a lens. The lens images the plane mirror of the Cassegrain system, which mirror defines a pupil of the imaging optical system, in the sectional circular area of the notional sphere, which extends around the apex of the cone defined by the funnel, and which passes through the borderlines of the detector elements. This prior art device, however, does not yet provide optimum signal-to-noise ratio. I have found that there is still stray radiation emanating from the utilized path of rays itself. Such stray radiation is emitted, in particular, from the reticle.
It is the object of the invention to further reduce the stray radiation in a detector device of the type previously discussed. In accordance with the invention this object is achieved in that the reticle is located in the vacuum chamber and in heat-conducting connection with the cooling means container. Thereby the reticle is cooled and the stray radiation emanating therefrom is thereby reduced. Accordingly, the image plane of the imaging path of rays has to be displaced into the vacuum chamber. Preferably the reticle is located in the radiation entrance aperture of said screen so that it is in heat-conducting connection with the cooling means container through this screen.